Sedimentary Facies Characteristics Of The Second Member Of Middle Jurassic Shaximiao Formation In Wubaochang Area,Northeast Sichuan Basin

It is one of the important shallow tight gas formations in Sichuan Basin of the second member of the Middle Jurassic Shaximiao Formation in Wubaochang area（referred to as the second member of Shaximiao Formation,J₂S₂）and it has good resource prospects.However,due to the inconsistent understanding of sedimentary facies types and unclear understanding of sedimentary facies distribution of J₂S₂ in Wubaochang area in the early stage,the further development of natural gas exploration is restricted.Based on the field outcrop data,drilling and logging data and seismic data of nearby Dukouhe area,Huanglongchang area,Qilibei area,Luojiazhai area and other areas,combined with the geological background,the sedimentary facies type and distribution of J₂S₂ is determined in Wubaochang area,the sedimentary boundary between subfacies is defined,the sedimentary evolution process is analyzed and the sedimentary facies model is established,the results show that:（1）From bottom to top,the second member of Shaximiao Formation in Wubaochang area can be divided into the submember of Shaximiao Formation 1（J₂S₂¹）,the submember of Shaximiao Formation 2（J₂S₂²）and the submember of Shaximiao Formation 3（J₂S₂³）,and the bottom boundary corresponds to No.1 sand body,No.5 sand body and No.8 sand body respectively.On the plane,the second member of Shaximiao Formation and its inner strata are thicker in the northeast and thinner in the southwest.（2）During J₂S₂ period in Wubaochang area develops meandering river delta deposition,with delta plain subfacies and delta front subfacies developed,which can be further subdivides into distributary channel,inter distributary channel,underwater distributary channel,inter distributary bay and estuary bar.（3）From the early stage to the late stage of J₂S₂ sedimentation in Wubaochang area,northeast Sichuan Basin,the water body gradually becomes shallower,and the study area gradually changes from the delta front deposition of J₂S₂¹to the delta plain deposition of J₂S₂³.According to the geological and seismic data,it is considered that:In the sedimentary period of J₂S₂¹,delta front deposits were developed,and the sand bodies were distributed in sheet-shaped and flower leaf shape,and their colors were mostly gray green.The pre-delta deposits are visible in the southern Huanglongchang area.The sedimentary boundary between delta front and pre-delta is located in Huanglongchang area and Luojiazhai Area.In the sedimentary period of J₂S₂²,delta plain deposits are developed in the north of the study area,and delta front deposits are dominant in the middle of the study area,and pre-delta deposits are dominant Huanglongchang local areas.The sedimentary boundary between delta front and pre-delta is near Huanglongchang area,and the sedimentary boundary between delta front and delta plain is in the line of well Qian 1-1-well Qian 15-well Qian 8-well Yangjia 1;In the sedimentary period of J₂S₂³,the lacustrine basin continues to shrink to the southwest,and delta plain deposits develope in the study area,with a large number of purple-purple red mudstone deposits visible,and part of the southern Huanglongchang area is delta front deposits.The sedimentary boundary between delta front and delta plain is located near Huanglongchang area and Luojiazhai area.（4）The sedimentary facies model of the delta progressive from northeast to southwest during J₂S₂ in the study area is established.It is comprehensively considered that channel sand bodies are relatively developed in the study area during the sedimentary period of J₂S₂¹and J₂S₂².Among them,the sand bodies are particularly developed in Wubaochang area and Dukouhe area,while the sand body is dominated by muddy deposition in J₂S₂³.